Yeast mitochondria have a circular duplex DNA coding for components of the translational machinery and for subunit polypeptides of the respiratory chain. The mitochondrial genome has been almost completely sequenced. While some of the genes are co-linear with the protein sequences, others are split with numerous intervening sequences. A case in point is the gene for cytochrome b which in the strain of yeast studied consists of three exons and two introns. One of the introns contains a long open reading frame capable of coding for a second protein that might have a function in the maturation of the cytochrome b messenger. In addition to providing information about the genetic content and gene organization of mitochondrial DNA, the sequence data has revealed that the genetic system of mitochondria differs in significant respects both in the codon usage and the codon recognition rules. During the next year we intend to make use of the DNA sequence data to examine the mechanism of processing and maturation of mitochondrial RNA transcribed from mosaic genes.